Manual S is the second half of doing sizing right. Where Manual J calculates how much cooling your house needs, ACCA's Manual S — Residential Equipment Selection (an ANSI-recognized national standard) tells the contractor how to pick the actual piece of equipment that delivers it. It is not guesswork: Manual S has the installer read the manufacturer's expanded performance data — how a given unit performs for sensible cooling, latent (humidity) removal, and capacity at your local design temperature, not at lab conditions.
The standard sets a clear sizing window: cooling equipment should be selected at roughly 95% to 115% of the design total cooling load from the Manual J. That guardrail is what prevents the comfort-and-cost problems of oversizing (short-cycling, poor dehumidification, higher bills, shorter equipment life) and undersizing (can't keep up in a heat wave). In a humid climate, the latent-capacity check matters as much as the raw tonnage — a unit can be "big enough" on paper and still leave a house damp if it was never selected against the latent load.
For a homeowner, Manual S is the reason two contractors with the same Manual J number can still propose different — and differently correct — equipment. A contractor who works to Manual S can explain why a specific model and stage configuration was chosen for your load and climate, rather than defaulting to the tonnage that happens to be in stock. Together with Manual J and Manual D (duct design), it is the documentation trail of a properly engineered replacement.